In recent years, progress has been made in research and development of diverse functional elements which involve use of an organic semiconductor.
One typical example of such functional elements is an organic EL element. The organic EL element, which is a current-driven light-emitting element, includes a pair of electrodes, i.e. an anode and a cathode, and a functional layer layered between the pair of electrodes, the functional layer containing an organic material. To drive the organic EL element, a voltage is applied to between the pair of electrodes to use the phenomenon of electroluminescence that occurs when holes injected from the anode into the functional layer recombine with electrons injected from the cathode into the functional layer. Being self-luminescent, the organic EL element is highly visible. In addition, being a complete solid-state element, the organic EL element has excellent impact resistance. Owing to these advantages, more attention is being given to the applications of organic EL elements as a light-emitting element or a light source for various display apparatuses.
To cause the organic EL element to emit light with low power consumption and high luminance, efficient injection of carriers (i.e., holes and electrons) from the electrodes to the functional layer is important. In general, to inject carriers efficiently, providing injection layers between each of the two electrodes and the functional layer for lowering energy barriers therebetween is effective. An injection layer disposed between the functional layer and the anode is a hole injection layer made of an organic material such as copper phthalocianine or PEDOT (electroconductive polymer), or a metal oxide such as tungsten oxide.
Also, an injection layer disposed between the functional layer and the cathode is an electron injection layer made of an organic material such as a metal complex or oxadiazole, a low work-function metal such as barium, or ion crystal such as sodium fluoride.
On the other hand, to realize longevity of the organic EL element, each layer constituting the organic EL element needs to be stable to the air. This is because many of the organic materials, low work-function metals and the like constituting the functional layer and electrodes are susceptible to oxygen and water contained in the air and deteriorate.
A generally adopted measure for the problem is to seal the whole organic EL element to avoid it from being exposed to the air. At the same time, however, development of functional layers that are stable even in the air environment is underway. It is known in particular that the electron injection layer made of titanium oxide can inhibit oxygen and water from entering into an adjacent functional layer and have relatively high resistance against oxygen and water in the electron injection layer itself (Non-Patent Literature 1, Patent Literature 1).